The antigen-specific activation of purified mouse B lymphocytes is being studied in a serum-free defined medium. Purified conventional antibodies or monoclonal antibodies from hybridomas directed against cell surface immunoglobulin M or D or non-immunoglobulin B lymphocyte specific surface antigens are being used to attempt to inhibit cell proliferation or differentiation to antibody secretion. The relative importance of different cell surface components in the process of cell activation thus is being studied. The cell surface phenotype of resting and activated B cells is simultaneously being evaluated using the fluorescence-activated cell sorter. Finally, clonally-restricted responses to phosphorylcholine are being evaluated in the in vitro defined system.